India-Asia Collision Zone Research Articles

Provenance Control on Chemical Weathering Index of Fluvio‐Lacustrine Sediments: Evidence From the Qaidam Basin, NE Tibetan Plateau

AbstractCarbon dioxide drawdown resulting from enhanced chemical weathering during orogenesis has been invoked to explain late Cenozoic global cooling. Establishing chemical weathering records from the India–Asia collision zone is important to test this hypothesis because uplift of the Tibetan Plateau is thought to be responsible for Cenozoic cooling. However, proxies for the intensity of chemical weathering can be affected by additional factors, such as sediment grain size and provenance. Here we report major element compositions and calculated chemical weathering intensity records of three size fractions (0–5, 5–20, 20–63 μm) from the Dahonggou section of the Qaidam Basin in the northeastern Tibetan Plateau, and compare those records with published provenance data from the same section. Results show that the indices of the fine (0–5 μm) fraction vary in coordination with provenance shifts, but variations of the 5–20 and 20–63 μm fractions are less affected by provenance variations. Comparison with Upper Continental Crust reveals that some labile elements are not leached but instead are enriched in the fine fraction, indicating that it does not faithfully record chemical weathering intensity. In addition, weathering can result in clay mineral transformation instead of elemental variations, complicating the relationship between element‐based parameters and weathering intensity. This work suggests that changes in sediment provenance must be accounted for when inferring variations in chemical weathering intensity on the basis of element‐based weathering intensity indices of the clay fraction.

Metamorphic Petrology of Clinopyroxene Amphibolite from the Xigaze Ophiolite, Southern Tibet: P-T Constraints and Phase Equilibrium Modeling

The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone (YZSZ), southern Tibet. The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite melange, four overprinted metamorphic stages are established. They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole, an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite, which is formed during burial process, an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite, and a decomposing retrograde stage M4, which is represented by the amphibolite+plagioclase symplectite+titanite+ rutile+quartz. By applying the THERMOCALC (versions 6.2 and 6.3) technique in the NCFMASHTO system, the P-T conditions estimated from M1 to M4 stages are ca. 8.6 kbar/880 °C, 10.8-13.4 kbar/800-840 °C, 12.7-13.2 kbar/650-660 °C and <11.2 kbar/640 °C, respectively. The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite, suggesting that the rocks underwent a cooling process during burial from magmatic protolith, and a decompressing stage after the pressure peak metamorphic conditions, which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes. The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies, granulite facies and eclogite facies with a burial depth of 30–40 km. After exhumation of the ophiolitic unit to the shallow crustal levels, the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams. A late subgreenschist facies overprinting subsequently occurs, the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.

Petrogenesis and tectonic implications of the Eocene-Oligocene potassic felsic suites in western Yunnan, eastern Tibetan Plateau: Evidence from petrology, zircon chronology, elemental and Sr-Nd-Pb-Hf isotopic geochemistry

In western Yunnan, potassic felsic suites with important Cu mineralization have been interpreted as being genetically associated with the Ailao Shan-Red River (ASRR) shear zone. In this paper, we show that the magmatism is unrelated to the shear zone, but shares many characteristics in common with potassic suites along the Jinsha suture to the north and northwest on the northern Tibetan Plateau, representing a magmatic response to the continued India-Asia convergence since the collision in the early Eocene. These felsic suites include syenite, monzogranite porphyries and granite porphyries. We dated 15 representative samples using zircon U-Pb method that gives a tight emplacement age of ~37–35 Ma. On 45 representative samples, we have done bulk-rock major element, trace element and Sr-Nd-Pb isotope analysis. We use these data, together with zircon Hf isotope data and the literature data on potassic mafic suites in the region to discuss the petrogenesis of these rocks. We conclude that the petrogenesis of the felsic potassic suites is most consistent with a scenario of deep (≥ 40 km) melting of thickened crust. The crustal melting is caused by intrusion and underplating of potassic mafic melts derived from melting of metasomatized mantle lithosphere. The mafic potassic suites in the region result from such mantle melts. Mixing of the mafic melts with the felsic melts explains the syenite and monzogranite porphyries and their extended fractional crystallization gives rise to the granite porphyries. The potassic suites along the Jinsha suture, including those from northern Tibet, western Yunnan and Vietnam, share the same tectonic setting during the Eocene-Oligocene. In response to the continued India-Asia convergence, removal of the lower continental lithospheric mantle triggered by Rayleigh-Taylor convective instability induces asthenosphere upwelling, partial melting of the metasomatized mantle lithosphere. We hypothesize that the eastward migration of the potassic magmatism along the Jinsha suture in the period of Eocene-Oligocene results from eastward thickening and removing of the continental lithospheric mantle along the suture, which, in time and space, correlates with the dischronal subduction of the torn Indian continental slab at ca. 52 Ma along the India-Asia collision zone.

Continental lithospheric-scale subduction versus crustal-scale underthrusting in the collision zone: Numerical modeling

Two distinct styles of continental collision are observed in nature, i.e. lithospheric-scale subduction versus crustal-scale underthrusting. The former refers to a collision scenario where the lower plate subducts below the lithospheric mantle of the overriding plate, whereas the latter is characterized by the underthrusting of the subducting plate beneath the crust of the overriding plate. Systematic numerical models were constructed to study the dynamics of these two different collision modes. The model results indicate that low convergence rate and cold overriding plate contribute to the lithospheric-scale subduction, which incorporates limited shortening of the overriding lithosphere during collision. In contrast, the crustal-scale underthrusting requires relatively high convergence rate and hot/weak overriding plate, in which the thickened lithospheric mantle of the overriding plate is progressively delaminated from the overlying crust and is replaced by the sub-horizontally underthrusted plate. The model results were systematically compared with four ongoing collisional orogens along the Alpine–Himalayan belt, i.e. the central-western Alps, the East Anatolian plateau, the Zagros–Iranian plateau, and the southern Tibetan plateau. It indicates that the lithospheric-scale subduction is widely observed and generally consistent with the natural orogens, for example, the Alps, Anatolia, and Zagros. On the other hand, the crustal-scale underthrusting is reconciled with the India–Asia collision zone, with respect to the geodynamic conditions, crustal-lithospheric structure, as well as the uplift history.

Present-day distribution of deformation around the southern Tibetan Plateau revealed by geodetic and seismic observations

The modern kinematics in and around the Tibetan Plateau have been widely investigated; however, the deformation distribution pattern describing the crustal kinematics in this region is controversial. Here, we quantify the deformation distribution in the southern Tibetan Plateau by adopting the partitioning ratio of internal to boundary potency rates instead of the individual slip rate data on major faults used in previous studies. The study area was divided into several blocks by major faults based on geological surveys, seismic data and a significance statistical analysis. For each block, the partitioning ratio of the potency rates was calculated by a microplate model using 294 GPS velocities from 13 studies. The deformation distribution derived from the GPS velocities was validated by surface-wave data from 407 earthquake events. After excluding the potency rates from the boundary between the Tibetan Plateau and Indian Plate, the ratio of intrablock potency rates within the southern Tibetan Plateau was 51%. This suggests that the Tibetan deformation is diffusely distributed over a large range. A seismic data analysis indicated that 53% of the historical seismic moments were released in the modeled blocks since 1901, which supported the conclusion based on the GPS velocities. The F-test indicated that the internal strain in most Tibetan blocks cannot be neglected to fit the surface velocities. Thus, the present-day crust in southern Tibet is probably far from a pure rigid block. This is supported by the relatively slow strike-slip rates of two major faults: the Karakorum fault (5.9 ± 0.8 mm yr−1) and Karakorum-Jiali fault (4.4 ± 0.8 mm yr−1 in the east segment, 9.3 ± 0.8 mm yr−1 in the west segment). The large-scale diffuse deformation in the southern Tibetan Plateau reproduced by the microplate model with significant internal strain, with seismic imaging or structural analyses, could further clarify the geodynamic processes in the India-Asia collision zone.

The burial and exhumation history of the Liuqu Conglomerate in the Yarlung Zangbo suture zone, southern Tibet: Insights from clumped isotope thermometry

The Liuqu Conglomerate, exposed along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet, archives not only the process of the India-Asia collision, but also the exhumation history of the India-Asia collision zone. The burial and exhumation history of the Liuqu Conglomerate thus could potentially provide clues to the India-Asia collision and orogeny. Low-temperature thermochronometric data indicate that maximum burial temperatures of the Liuqu Conglomerate ranged from 80 to 110 °C; however, the exact burial history is not well constrained. Here, we examine the burial conditions of the Liuqu Conglomerate by applying clumped isotope thermometry to paleosol carbonates. Extensive microcrystalline recrystallization indicates that the samples have been diagenetically altered during burial. Moreover, the clumped isotope temperatures (ranging from ∼48 to ∼97 °C) are clearly above Earth surface conditions, indicating the original climatic information was overwritten by alteration at higher burial temperatures. Nevertheless, the calculated diagenetic water δ18O values represent maximum meteoric water values that can be used to broadly constrain the paleoelevation. The highest clumped isotope temperature implies that the maximum burial temperature of the Liuqu Conglomerate was at least ∼97 °C. We conclude that the Liuqu Conglomerate was buried as deeply as 3.7–4.3 km during the latest Paleocene to Eocene, and then exhumed during the Miocene (∼10–12 Ma). Incision of the paleo-Yarlung River was likely responsible for erosion and exhumation of the Liuqu Conglomerate, which suggests that the drainage system of Paleo-Yarlung River was similar to that of today and supports the model of outward growth of the Tibetan Plateau.

Paleogene evolution of the Burmese forearc basin and implications for the history of India-Asia convergence

The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene – Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin - the northern part of the Burmese forearc - and integrate these results with previous data from other basins in central Myanmar. Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late Middle Eocene, with a forearc basin open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar was formed 39-37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern precollisional paleogeography for the Indochina Peninsula, and indicate that any postcollisional rotation of Indochina, if it occurred at all, must have been achieved by the late Middle Eocene.

Structural and metamorphic evolution of the Karakoram and Pamir following India–Kohistan–Asia collision

Abstract Following the c. 50 Ma India–Kohistan arc–Asia collision, crustal thickening uplifted the Himalaya (Indian Plate), and the Karakoram, Pamir and Tibetan Plateau (Asian Plate). Whereas surface geology of Tibet shows limited Cenozoic metamorphism and deformation, and only localized crustal melting, the Karakoram–Pamir show regional sillimanite- and kyanite-grade metamorphism, and crustal melting resulting in major granitic intrusions (Baltoro granites). U/Th–Pb dating shows that metamorphism along the Hunza Karakoram peaked at c. 83–62 and 44 Ma with intrusion of the Hunza dykes at 52–50 Ma and 35 ± 1.0 Ma, and along the Baltoro Karakoram peaked at c. 28–22 Ma, but continued until 5.4–3.5 Ma (Dassu dome). Widespread crustal melting along the Baltoro Batholith spanned 26.4–13 Ma. A series of thrust sheets and gneiss domes (metamorphic core complexes) record crustal thickening and regional metamorphism in the central and south Pamir from 37 to 20 Ma. At 20 Ma, break-off of the Indian slab caused large-scale exhumation of amphibolite-facies crust from depths of 30–55 km, and caused crustal thickening to jump to the fold-and-thrust belt at the northern edge of the Pamir. Crustal thickening, high-grade metamorphism and melting are certainly continuing at depth today in the India–Asia collision zone.

Timescales of collisional metamorphism from Sm-Nd, Lu-Hf and U-Pb thermochronology: A case from the Proterozoic Putumayo Orogen of Amazonia

The construction of mountain belts by continent-continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction-driven plate tectonics on Earth. Yet, quantitative estimation of the rate and duration of prograde orogenic metamorphism in high-grade collisional belts has proven elusive to constrain. Differences in the chemical partitioning and diffusive behavior of elements associated with the 147Sm-143Nd and 176Lu-176Hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes. Here, we present new Sm-Nd and Lu-Hf isotopic results from multiple garnet and whole-rock aliquots from a granulite-grade metasedimentary rock from the Proterozoic Putumayo Orogen of South America. Using a combination of internal isochron dating by these two decay systems, combined with apatite U-Pb dating and pressure-temperature-time (P-T-t) constraints from mineral equilibria and chemical diffusion geospeedometry, we provide first-order insights into the time scales of a Proterozoic continent-continent collisional metamorphic event in Amazonia associated with its incorporation to the Supercontinent Rodinia. Our results indicate that these metasediments attained peak granulite-grade metamorphic conditions of nearly 0.62 GPa and 680 °C at approximately 1035 Ma, and started to cool slowly, presumably during exhumation, at a rate of ∼5 K/Myr. Simple diffusion scaling relationships and numerical modeling of Sm-Nd and Lu-Hf in garnet indicate that, while Sm and Nd were both fully re-equilibrated at peak temperatures and therefore date cooling, Lu and Hf underwent diffusive decoupling due to incomplete Hf re-equilibration. By modeling the impact of this differential re-equilibration in the development of the Sm-Nd and Lu-Hf isochrons, we constraint the duration of the prograde metamorphic path and argue that garnet growth in this sample took place over millions of years—approximately 25 Myr according to our preferred interpretation. The determined apatite U-Pb date vs. grain-size relationship cannot be reconciled with the thermal history described above when modeled using the existing experimental diffusion data, and instead suggest that much slower Pb diffusivities in this mineral compared to those predicted by the existing experimental data may have inhibited its resetting. We note that the inferred temperature-time history for the metamorphic loop of this segment of the Putumayo basement is comparable to that proposed for the metamorphism and exhumation of metasediments in the India-Asia collision zone, suggesting that similar tectonic processes and rates likely operated during the Mesoproterozoic Putumayo orogeny.

Tectonic evolution of the NE section of the Pamir Plateau: New evidence from field observations and zircon U-Pb geochronology

The Pamir Plateau at the western end of the India–Asia collision zone underwent long-term terrane drifting, accretion and collision between early Paleozoic and Mesozoic. However, the detailed evolution of this plateau, in particular, the timing of the Proto- and Palaeo-Tethys ocean subduction and closure, remains enigmatic. Here we report new field observations and zircon U-Pb ages and Hf isotopic compositions of the representative rocks from the so-called Precambrian basement in the northeastern Pamir, i.e., the Bulunkuole Group. The rock associations of the Bulunkuole Group indicate volcano-sedimentary sequences with arc affinities. Geochronological data demonstrate that the deposition age of the Bulunkuole Group in the NE section of the Pamir was Middle to Late Cambrian (530–508Ma) rather than Paleoproterozoic. The deposition age became progressively younger from south to north. The amphibolite- to granulite facies metamorphism of the Bulunkuole Group took place at ca. 200–180Ma. Unlike the scenario in the Southern Kunlun terrane (SKT) in the eastern section of the West Kunlun Orogenic Belt (WKOB), early Paleozoic metamorphism (ca. 440Ma) was absent in this area. Two phases of magmatic intrusions, composed of granites and minor gabbros with arc geochemical signatures, emplaced at 510–480Ma and 240–200Ma. The amphibolite (meta mafic sheet? 519Ma) and the meta-rhyolite (508Ma) have zircon εHf(t) values of 1.6 to 5.9 and −1.5 to 1.4, respectively. The 511Ma gneissic granite sheet and the 486Ma gabbro have zircon εHf(t) values of −0.1 to 2.4 and 1.3 to 3.6, respectively. Zircon εHf(t) of the 245Ma augen gneissic granite sheet varies from −2.2 to 2.0 whereas the metamorphic zircons from the amphibolite (193Ma) and high-pressure mafic granulite sample (187Ma) have negative εHf(t) values of −5.3 to −2 and −15 to −12, respectively. In line with rock association and the deposition age of the Bulunkuole Group and the Saitula Group in the eastern section of WKOB, we propose that both of them were accretionary wedge between the Tarim and Tianshuihai terrane formed during the Proto-Tethys ocean south- southwestward subduction (present orientation). The timing of deposition and metamorphism documents two distinct phases of arc magmatism and sedimentary basin evolution. This indicates that the Proto-Tethys ocean closed at ca. 440Ma in the eastern section of WKOB whereas in the western section, a remnant of the Proto-Tethys ocean between the Tarim and the NE Pamir did not close till Late Triassic. This remnant ocean was filled by the Ordovician-Triassic sequences. The accretion between NE Pamir and the Central Pamir was completed by ca. 180Ma as demonstrated by the metamorphic zircon U-Pb age of the high pressure mafic granulite in the NE Pamir.

Crustal rheology controls on the Tibetan plateau formation during India-Asia convergence

The formation of the Tibetan plateau during the India-Asia collision remains an outstanding issue. Proposed models mostly focus on the different styles of Tibetan crustal deformation, yet these do not readily explain the observed variation of deformation and deep structures along the collisional zone. Here we use three-dimensional numerical models to evaluate the effects of crustal rheology on the formation of the Himalayan-Tibetan orogenic system. During convergence, a weaker Asian crust allows strain far north within the upper plate, where a wide continental plateau forms behind the orogeny. In contrast, a stronger Asian crust suppresses the plateau formation, while the orogeny accommodates most of the shortening. The stronger Asian lithosphere is also forced beneath the Indian lithosphere, forming a reversed-polarity underthrusting. Our results demonstrate that the observed variations in lithosphere deformation and structures along the India-Asia collision zone are primarily controlled by the strength heterogeneity of the Asian continental crust.

Clockwise rotations recorded in redbeds from the Jinggu Basin of northwestern Indochina

Paleomagnetic data have been used to infer clockwise rotations and significant southward motion of the Indochina block during Cenozoic extrusion from the India-Asia collision zone. Because the Cenozoic of the Indochina block has been sparsely sampled to date and is key to determining the timing of this tectonic motion, we performed an extensive paleomagnetic study on Paleocene to Oligocene redbeds and middle Miocene sandy silts from the Jinggu Basin (23.5°N, 100.7°E), in northern Indochina. Paleomagnetic results from the redbeds pass fold tests, but they show exclusively normal polarity. There is controversy, however, on the age assignment to the lower part of the succession. If the age of the redbeds is indeed Paleogene, this indicates a prefolding remagnetization associated with Oligocene deformation of the Indochina block. If the age of the redbeds would be Late Cretaceous, their magnetization may be primary, and deposition could have taken place during the Cretaceous normal superchron. However, the abundance of secondary hematite in the redbeds in combination with the biostratigraphy, which indicates a Paleogene age for at least the upper two formations of the redbeds in the Jinggu Basin, implies a pervasive remagnetization. The middle Miocene sediments pass both the fold test and the reversals test and contain magnetite as well as hematite as carriers, suggesting a primary magnetization. Our large data set from the redbeds (>2000 paleomagnetic directions) demonstrates an ∼30°−35° clockwise rotation of the Jinggu Basin with respect to Eurasia, for both scenarios, i.e., when compared to a ca. 100 Ma pole (if the redbeds carry a primary natural remanent magnetization) or to a ca. 30 Ma pole (if the sequence is remagnetized). The middle Miocene results, however, indicate that the Jinggu Basin experienced no significant (2° ± 5.6°) rotation with respect to Eurasia. Since no major deformation has occurred within northern Indochina during Late Cretaceous to Eocene times, our results reflect a major clockwise rotation of the Indochina block during its Oligocene to early Miocene extrusion from the India-Asia collision zone.

Seismotectonics of the Trans-Himalaya, Eastern Ladakh, India: Constraints from moment tensor solutions of local earthquake data

The seismotectonic scenario of the northwest part of the India-Asia collision zone is studied by analyzing the local earthquake data (M~1.4–4.3) recorded by a broadband seismological network consisting of 14 stations. Focal Mechanism Solutions (FMSs) of 13 selected earthquakes were computed through waveform inversion of three-component broadband records. Depth distribution of the earthquakes and FMSs of local earthquakes obtained by waveform inversion reveal kinematics of the major fault zones present in eastern Ladakh. A most pronounced cluster of seismicity is observed in the Karakoram Fault (KF) zone down to a depth of ~65km. The FMSs reveal a transpressive environment with an inferred strike slip fault plane parallel to the KF. It is argued that the KF penetrates down to the lower crust and is a manifestation of active underthrusting of Indian lower crust beneath Tibet. Two clusters of microseismicity are observed at a depth range 5–20km at the northwestern and southeastern fringes of the Tso Morari gneiss dome, which can be correlated to the activities along the Zildat fault and Karzok fault, respectively. The FMSs obtained for representative earthquakes show thrust fault solutions for the Karzok fault, and normal fault solutions for the Zildat fault. It is suggested that the Zildat fault is acting as a detachment, facilitating the exhumation of the Tso Morari dome. On the other hand, the Tso Morari dome is underthrusting the Karzok ophiolite at its southern margin along the Karzok fault due to gravity collapse.

Late Permian to Early Oligocene granitic magmatism of the Phan Si Pan uplift area, NW Vietnam: their relationship to Phanerozoic crustal evolution of Southwest China

The Phan Si Pan uplift area of NW Vietnam is a part of the Archean to Paleoproterozoic Yangtze Block (SW China) that was displaced along the Ailaoshan-Red River fault shear zone and adjacent structures (i.e., Song Da zone). This area is of particular interest because it experienced a number of Phanerozoic crustal building events, including the Emeishan large igneous province, the India-Eurasia collision, and Ailaoshan – Red River Fault displacement. In the Phan Si Pan uplift area, there are at least three different geochronological complexes, including (1) Late Permian, (2) Eocene, and (3) Early Oligocene. (1) The Late Permian silicic plutonic and volcanic rocks are correlated with the Emeishan large igneous province (ELIP). The silicic rocks of the Phan Si Pan uplift area, and Tu Le basin are ferroan alkalic and have zircon CA-ID-TIMS weighted-mean ages of 256.3 ± 0.4 Ma to 257.3 ± 0.1 Ma for granite and 257.1 ± 0.6 Ma to 257.9 ± 0.3 Ma for rhyolite. The high precision CA-ID-TIMS of Phan Si Pan – Tu Le silicic rocks suggest that Emeishan lavas erupted over a period of ~6 million years with plutonism ending shortly thereafter. Granitic rocks from Phan Si Pan uplift and Tu Le basin have variable ISr values ranging between 0.70214 and 0.70357 that could be the result of significant fractional crystallization (i.e., highly depleted Sr) or mobility of Rb, and eNd(t) values are broadly chondritic (eNd(t) = 0 ± 1). Zircon eHf(t) values range from +3.3 to +8.8 suggesting the granitic rocks of Phan Si Pan uplift and Tu Le basin are derived from a mantle source. Reported fractional crystallization modeling suggests the Phan Si Pan – Tu Le silicic rocks can be produce from Emeishan high-Ti basaltic rocks. (2) The Ban Xeo granite is ferroan (A-type), alkalic, and yielded a zircon weighted-mean 206Pb/238U age of 49 ± 0.9 Ma. The major and trace element compositions of the Ban Xeo granite are similar to the spatially associated Late Permian Muong Hum granites. The eNd(t) values (-2.5 to -1.4), 87Sr/86Sr initial ratios (0.70793, 0.70989, and 0.75341), and zircon eHf(t) values (-1.1 to +3.0) indicate that the Ban Xeo granite is likely derived by differentiation of a mafic parental magma that had EMII-like mantle source characteristics. (3) The younger Ye Yen Sun (YYS) granite is magnesian, alkali-calcic to calc-alkalic, and yielded zircon U-Pb weighted-mean ages of 35.3 ± 0.5 to 34.1 ± 0.4 Ma. The YYS granites have 87Sr/86Sr initial ratios of 0.70636 to 0.70860, eNd(t) values from -8.7 to -2.6, and zircon eHf(t) values from -9.3 to +0.6 which indicate a crustal origin. The YYS dioritic rocks range from syeno-diorite to granodiorite, and have similar Sr (87Sr/86Sr = 0.70627) and eNd(t) (-3.8) values as the YYS granites. However, the zircon Hf isotopes (eHf(t) = +0.3 to +3.2) of the syeno-diorite are slightly more chondritic than the granites and indicates that the parental magma was likely derived from a mantle source. Rhyolite–MELTS modeling results show that the YYS granites were likely generated by fractionation of a parental magma similar in composition to the granodiorite. Furthermore, the YYS syeno-diorites may be representative of a cumulate rock that developed during the early stages of emplacement or crystallization. The parental magma of YYS igneous rocks likely formed by partial melting of mafic Mesoproterozoic juvenile rocks (lower crust/uppermost mantle) by heat from the upwelling lithospheric mantle during a period of crustal relaxation. The formation of the Ban Xeo granites and Ye Yen Sun igneous rocks are likely related to lateral melt migration along the eastern edge of the India-Asia collision zone. It is possible that repeated episodes of Cenozoic magmatism was pivotal in the development and initiation of fault movement of the Ailaoshan-Red River shear zone.

Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia)

Subduction of buoyant continental lithosphere is one of the least understood plate-tectonic processes. Yet under the Pamir–Hindu Kush, at the northwestern margin of the India–Asia collision zone, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. Here, we report new precise earthquake hypocenters, detailed tomographic images and earthquake source mechanisms, which allow distinguishing a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir. We suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India's thinned western continental margin separates from Cratonic India and subducts beneath Asia. This torn-off narrow plate sliver forms a prominent high-velocity anomaly down to the mantle transition zone. Our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab, providing a rare glimpse at the ephemeral process of slab break-off.

Magmatic record of India-Asia collision.

New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80–40 Ma migrated from south to north and then back to south with significant mantle input at 70–43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52–51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma.

Post-collisional Ultrapotassic Mafic Magmatism in South Tibet: Products of Partial Melting of Pyroxenite in the Mantle Wedge Induced by Roll-back and Delamination of the Subducted Indian Continental Lithosphere Slab

Post-collisional (25–8 Ma) ultrapotassic mafic magmatic rocks occur to the north of the India–Asia collision zone within the Lhasa terrane of the southern Tibetan Plateau, forming a near 1000 km long semi-continuous igneous belt. They include both extrusive and intrusive facies, although lava flows dominate. To understand their petrogenesis, the mineral chemistry of olivine phenocrysts and xenocrysts and whole-rock major and trace element and Sr–Nd–Pb isotope data are presented for the most primitive mafic magmatic rocks (MgO > 6 wt %) from west to east. The studied samples are characterized by high MgO (6·28–15·75 wt %), K2O (4·76–8·89 wt %), SiO2 (46·44–59·74 wt %), Ba (1368–14076 ppm), Th (69–336 ppm) and Ni (106–527 ppm) contents. Chondrite-normalized rare earth element (REE) patterns show enrichment in light rare earth elements (LREE), flat heavy REE (HREE) patterns and negative Eu anomalies. These REE patterns have a very distinctive inverted ‘spoon shape’, which appears to be a common characteristic of collision-related ultrapotassic magmas. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and strong negative Ta–Nb–Ti anomalies, which are typical of subduction-related magmas. The ultrapotassic magmatic rocks studied have extremely radiogenic initial Sr isotopic compositions (0·712379–0·737616) and low (143Nd/144Nd)i (0·511662–0·511984). Combined with their Pb isotope compositions [(206Pb/204Pb)i = 18·30–18·92; (207Pb/204Pb)i = 15·65–15·87; (208Pb/204Pb)i = 39·02–39·76] these data are consistent with the involvement of a subducted continental crustal component in their petrogenesis. The Sr–Nd–Pb isotope compositions exhibit linear trends between depleted mid-ocean ridge basalt (MORB)-source mantle (DMM) and Indian continental crust. The extreme enrichment of the upper mantle below south Tibet is considered to result from the addition of components derived from subducted Indian continental crust to the overlying mantle wedge during northward underthrusting of Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ∼55 Ma. The post-collisional K-rich mafic magmas in south Tibet were generated by partial melting of pyroxenite in a mantle source region that was created by reaction of hydrous fluids and siliceous melts from subducted granulite–eclogite-facies Indian continental crustal rocks with the surrounding peridotitic mantle. A continuous process from slab roll-back, through break-off, to detachment of the slab may have induced partial melting of the pyroxenites. Cessation of the post-collisional ultrapotassic magmatism at ∼8 Ma may be linked to the onset of flat slab subduction beneath southern Tibet and the elimination of the wedge of Tibetan subcontinental lithospheric mantle and underlying asthenosphere; geophysical data indicate that at the present day eclogite-facies Indian continental crust directly underthrusts the crust of the Lhasa terrane with no intervening mantle wedge. The proportion of the Indian continental crustal component in the mantle source of the ultrapotassic mafic magmas decreases eastward, as do the ages and volumes of the magmatic rocks. There are no outcrops of post-collisional K-rich mafic magmatic rocks (MgO > 6 wt %) to the east of 87°E in the Lhasa terrane, which may indicate a change in subduction geometry at this longitude.

Constraining the age of Liuqu Conglomerate, southern Tibet: Implications for evolution of the India–Asia collision zone

Controversy over the depositional age and provenance of the Liuqu Conglomerate along the major structural Indus–Yarlung suture zone in South Tibet clouds our understanding of the process of the India/Asia collision. Here, we report low-temperature thermochronometric data (apatite fission track, apatite and zircon (U–Th)/He for the Liuqu Conglomerate in the Xigaze area). Our new data constrain its depositional age to latest Oligocene–Early Miocene time, indicating that rather than having formed immediately following Paleogene India–Asia collision or collision between India and an intra-oceanic arc as previously proposed, the Conglomerate was probably deposited in an intermontane basin, at a slightly later time than the Gangrinboche Group to the north. The Liuqu Conglomerate should therefore not be used as a key horizon in models constraining the early stages of India/Asia collision. Our data together with previous studies suggest that the Liuqu Conglomerate was sourced from the Xigaze forearc basin, Indus–Yarlung suture zone, as well as the Tethyan Himalaya. Furthermore, our data indicate that exhumation of the Liuqu Conglomerate commenced at ∼<10–12 Ma, suggesting significant erosion in the Indus–Yarlung suture zone attributable to incision of the Yarlung Zangbo in Mid to Late Miocene time.

Late Quaternary faulting on the Manas and Hutubi reverse faults in the northern foreland basin of Tian Shan, China

The Tian Shan Range lies in the actively deforming part of the India–Asia collision zone. In the northern foreland basin of Tian Shan, the strata were intensively deformed by Cenozoic folding and faulting. Slip rate studies along these faults are important for understanding the dynamics of crustal deformation and evaluating the seismic hazards in the region. Two reverse faults (the Manas and Hutubi faults) in the northern foreland basin were investigated. Due to past faulting events along these faults, the terrace treads along the Manas River were ruptured, forming fault scarps several meters in height. Loess deposits were trapped and preserved at the surface ruptures along these scarps. The thickness of the trapped loess is dependent on the size of the ruptures. The minimum and maximum ages of these scarps are constrained by dating the loess preserved at the surface ruptures and the terrace treads, respectively, using the quartz optically stimulated luminescence (OSL) dating technique. Our dating results suggest that the loess trapped at the ruptures was deposited from the early to mid-Holocene at the Hutubi Fault, and from the mid- to late-Holocene at the Manas Fault. The vertical displacements of the faults were evaluated by measuring the topographic profiles across the investigated fault scarps using the differential global position system (DGPS). Our results suggest that, during the late Quaternary in the studied region, the vertical slip rates of the Manas Fault were between ∼0.74 mm/yr and ∼1.6 mm/yr, while the Hutubi Fault had a much lower vertical slip rate between ∼0.34 mm/yr and ∼0.40 mm/yr. The tectonic implications of our results are discussed.

Low effective fault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone, NW India

Phyllosilicate-rich fault rocks are common in large-scale fault zones and can dramatically impact fault rheology. Experimental evidence suggests that multi-mechanism frictional-viscous flow (FVF) may operate in such lithologies, potentially significantly weakening mature fault cores. We report microstructures indicative of FVF in exhumed phyllonites of the Karakoram Fault Zone (KFZ), NW India. These include interconnected muscovite foliae, lack of quartz/feldspar crystal preferred orientations, and sutured grains and overgrowths indicative of fluid-assisted diffusive mass transfer. FVF microphysical modelling, using microstructural observations from the natural fault rock and experimentally-derived friction and diffusion coefficients, predicts low peak shear strengths of <20 MPa within the frictional-viscous transition zone. Chlorite geothermometry indicates that synkinematic chlorites grew at 351 ± 34 °C (c. 10 km depth) during FVF, immediately above the transition to quartz crystal plasticity. The deformation processes and interpreted low shear strength of the exhumed KFZ fault rocks provide analogues for processes operating currently at depth in active faults of similar scale. If similar lithologies are present at depth, then the Quaternary seismic characteristics of the KFZ support faults with phyllonitic cores being able to accommodate large seismic ruptures. The results also provide rare rheological constraints for mechanical models of the India-Asia collision zone.